Learning Objectives. Your goals in studying this chapter are to:

Transcription

1 Learning Objectives Your goals in studying this chapter are to: Understand how tornadoes form. Understand the factors that favor tornado formation. Understand the EF damage scale and the types of damage a tornado can do. Understand where and when tornadoes form. Understand how tornadoes are forecast. Understand tornado safety measures. Understand tornado watches and warnings. Understand basic tornado facts and statistics. Understand tornado hazards areas in the United States. (NASA)

2 Tornado Damage and Rating Throughout this text, if a link doesn t work, just do a web search on the phrase plus site:noaa.gov How is tornado damage rated? The most widely used method worldwide, for over three decades, was the F-scale developed by Dr. T. Theodore Fujita. In the U. S., and probably elsewhere within a few years, the new Enhanced F-scale is becoming the standard for assessing tornado damage. In Britain, there is a scale similar to the original F-scale but with more divisions. The wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically verified in real tornadoes. Enhanced F-scale winds are derived from engineering guidelines but still are only judgmental estimates because: 1. Nobody knows the "true" wind speeds at ground level in most tornadoes, and 2. The amount of wind needed to do similar-looking damage can vary greatly, even from block to block or building to building, Damage rating is (at best) an exercise in educated guessing. Even experienced damage-survey meteorologists and wind engineers can and often do disagree among themselves on a tornado's strength. What is the Enhanced F-scale? The Enhanced F-scale (simple table) is a much more precise and robust way to assess tornado damage than the original. It classifies F0-F5 damage as calibrated by engineers and meteorologists across 28 different types of damage indicators (mainly various kinds of buildings, but also a few other structures as well as trees). The idea is that a "one size fits all" approach just doesn't work in rating tornado damage, and that a tornado scale needs to take into account the typical strengths and weaknesses of different types of construction. This is because the same wind does different things to different kinds of structures. In the Enhanced F-scale, there will be different, customized standards for assigning any given F rating to a well built, well anchored wood-frame house compared to a garage, school, skyscraper, unanchored house, barn, factory, utility pole or other type of structure. In a real-life tornado track, these ratings can be mapped together more smoothly to make a damage analysis. Of course, there still will be gaps and weaknesses on a track where there was little or nothing to damage, but such problems will be less common than under the original F-scale. As with the original F-scale, the enhanced version will rate the tornado as a whole based on most intense damage within the path. There are no plans to systematically re-evaluate historical tornadoes using the Enhanced F-scale. A full PDF document on the Enhanced F-scale is online.

3 The Fujita scale can be generalized this way: F0 (Gale) F1 (Weak) F2 (Strong) F3 (Severe) F4 (Devastating) F5 (Incredible) But the table here is more precise. To determine F scale, determine amount of damage (columns) and the appropriate structure (rows). The intersection is the initial estimate then add or subtract the first column number.

4 EF1 Damage minor damage to strong frame house

5 EF2 Damage roof gone, strong frame house

6 EF3 Damage walls collapse, strong frame house or brick structure

7 EF4 Damage strong frame house or brick structure blown down Red arrows indicate EF4 damage

8 EF5 Damage strong frame house or brick structure blown away

9 How do tornadoes do some weird things, like drive straw into trees, strip road pavement and drive splinters into bricks? The list of bizarre things attributed to tornadoes is almost endless. Much of it is folklore; but there are some weird scenes in tornado damage. Asphalt pavement may strip when tornado winds sandblast the edges with gravel and other small detritus, eroding the edges and causing chunks to peel loose from the road base. Storm chasers and damage surveyors have observed this phenomenon often after the passage of a violent tornado. With a specially designed cannon, wind engineers at Texas Tech University have fired boards and other objects at over 100 mph into various types of construction materials, duplicating some of the kinds of "bizarre" effects, such as wood splinters embedded in bricks. Intense winds can bend a tree or other objects, creating cracks in which debris (e.g., hay straw) becomes lodged before the tree straightens and the crack tightens shut again. All bizarre damage effects have a physical cause inside the roiling maelstrom of tornado winds. We don't fully understand what some of those causes are yet, however; because much of it is almost impossible to simulate in a lab. I've heard about tornadoes picking up objects and carrying them for miles. Does this happen? Yes, numerous tornadoes have lofted (mainly light) debris many miles into the sky, which was then blown by middle- and upper-atmospheric winds for long distances. The vertical winds in tornadoes can be strong enough to temporarily levitate even heavy objects if they have a large face to the wind or flat sides (like roofs, walls, trees and cars), and are strong enough to carry lightweight objects tens of thousands of feet high. Though the heaviest objects, such as railroad cars, can only be airborne for short distances, stories of checks and other papers found over 100 miles away are often true. The Worcester MA tornado of 9 June 1953 carried mattress pieces high into the thunderstorm, where they were coated in ice, before they fell into Boston Harbor. Pilots reported seeing debris fluttering through the air at high altitude near the thunderstorm which spawned the Ruskin Heights MO tornado of 20 May How can a tornado destroy one house and leave the next one almost unscratched? Most of the time, this happens either with multiple-vortex tornadoes or very small, intense single-vortex tornadoes. The winds in most of a multivortex tornado may only be strong enough to do minor damage to a particular house. But one of the smaller embedded subvortices, perhaps only a few dozen feet across, may strike the house next door with winds over 200 mph, causing complete destruction. Also, there can be great differences in construction from one building to the next, so that even in the same wind speed, one may be flattened while the other is barely nicked. For example, a flimsy, unanchored mobile home may be obliterated while all surrounding objects suffer little or no damage.

10 (NSF)

11 Note how close undamaged houses are to ones that were completely blown away. Damage in the center of the tornado path is rated EF5, and tapers to lower ratings toward the outsides. (NOAA)